1. Field of the Invention
The present invention relates to a dynamic bearing device used as a bearing for a motor such as a spindle motor.
2. Background
Conventionally, as this type of dynamic bearing device, one as shown in FIG. 6 or FIG. 7 is known.
As shown in FIG. 6, this type of dynamic bearing device is known as one composed of a cylindrical radial dynamic pressure bearing 2 and a flange-shape thrust dynamic bearing 2b which follows the radial dynamic pressure bearing 2. A rotary member 3 is radially supported by the radial dynamic pressure bearing 2, whereas the rotary member 3 is axially supported by the thrust dynamic pressure bearing 2b.
As shown in FIG. 6, grooves 2c are formed in the circumferential direction of the bearing surface of the radial dynamic bearing 2, whereas grooves (not shown) are also formed in the bearing surface of the thrust dynamic pressure bearing 2b in the same manner. Oil is filled in a gap 5 between the radial dynamic pressure bearing 2, the thrust dynamic bearing 2b and the rotary member 3.
Further, FIG. 7 shows the detailed construction of the dynamic pressure bearing device. As shown in FIG. 7, this dynamic pressure bearing device has a stationary base 1. A lower portion of a cylindrical radial bearing portion 2 is fitted and fixed to a center of a stationary base 1. A thrust bearing portion 2b is formed on the upper portion of this radial bearing portion 2.
A rotary member composed of a bottomed rotor 3, a thrust retaining member 4 and the like is rotatably supported by the radial bearing portion 2 and the thrust dynamic pressure bearing 2b. A gap 5 for retaining the oil is formed between the rotary member, the radial bearing portion 2 and the thrust dynamic pressure bearing 2b.
As shown in FIG. 7, a magnet 6 is fixed to an inner circumferential surface of the bottomed rotor 3, and a motor is defined between this magnet 6 and a stator 7 formed by winding wirings around an iron core fixed to the side of the stationary base 1.
An annular oil sump 9 is formed between the lower surface of the bottomed rotor 3 and the stationary base 1 to be in communication with the gap 5, as shown in FIG. 7. Following the oil sump 9, a space 8 with its central portion slanted from the central portion in the radial direction is formed between the lower surface of the bottomed rotor 3 and the upper surface of the stationary base 1.
An oil feed hole 2a is formed in the axial direction of the center of the thrust dynamic pressure bearing 2. The oil that has been fed from the oil feed hole 2a is filled from a final end of the oil feed hole 2a to the gap 5. The overflown oil from the gap 5 is to be received into the oil sump 9.
According to the dynamic pressure bearing having such a structure, when the rotary member including a bottomed rotor 3 is started for rotation, due to the viscosity of the oil, the oil is entrained into the narrow portions of the gap 5 and compressed to increase the pressure of the oil. Thus, the pressure is on the balance with the weight of the rotary member to support the rotary member and to thereby form oil films in the gap 5. Thus, the rotary member is supported in a non-contact state to the radial dynamic pressure bearing 2 and the thrust dynamic pressure bearing 2b by the dynamic pressure generated by the rotation of the rotary member.
However, as shown in FIG. 7, the oil sump 9 is formed between the lower surface of the bottomed rotor 3 and the upper surface of the stationary base 1, and the space 8 is formed in communication with this oil sump 9.
Since the vertical width of the space 8 is very small, even if the bottomed rotor 3 is stopped, the oil is raised to the bottom portion of the space 8 by the capillary action.
On the other hand, upon the rotation of the bottomed rotor 3, since the centrifugal force caused by the rotation is applied to the interior of the space 8, the oil is further raised through the space 8. As a result, there is a fear that the raised oil would be leaked from the opening portion of the space 8.
Further, in the conventional dynamic pressure bearing device shown in FIG. 7, in order to miniaturize and thin the rotary member 3, it is necessary to shorten the radial dynamic pressure bearing 2.
However, even if the radial dynamic pressure bearing 2 is simply shortened without changing the conventional design, it is impossible to obtain a sufficient dynamic pressure in the radial direction of the rotary member 3, and it would not be possible to meet the requirements of the miniaturization and flatness.